1. Field of the Invention
The present invention relates to an upper arm made by pressing and used in a suspension system for a vehicle.
2. Description of Relevant Art
A so-called A-shaped upper suspension arm is conventionally known. The conventional arm is integrally formed by pressing a metal blank into a substantially U-shape or V-shape as viewed in a plane, and has bifurcated base ends pivotally supportable at respective front and rear points on a vehicle body frame and a tip end pivotally supportable at a single point on a knuckle (see Japanese Utility Model Publication No. 2804/89).
FIGS. 7 to 10 illustrate such known upper arm made by pressing. FIG. 7 is a plan view of the upper arm. FIG. 8 is a side elevational view taken in a direction of arrow 8 in FIG. 7. FIG. 9 is a sectional view taken along a line 9--9 in FIG. 7. FIG. 10 is a sectional view taken along a line 10--10 in FIG. 7.
The upper arm UA includes a body 1 made of a substantially U-shaped plate as viewed in a plane. A mounting hole 1.sub.1 for ball joint J.sub.1 for connecting the upper arm to a knuckle is provided by a downward burring in the body 1 at a location adjacent the tip end of the upper arm UA. An inner flange 2 extending downwardly and then extending inwardly is integrally formed by pressing at an inner edge of the body 1, i.e., at an edge surrounding a damper D and a suspension spring S which are disposed coaxially with each other, and an outer flange 3 extending downwardly and then extending outwardly is integrally formed by pressing at an outer edge of the body 1, i.e., at an edge remote from the damper D and the suspension spring S. Mounting holes 3.sub.1, 3.sub.1 for a pair of rubber bush joints J.sub.2, J.sub.2 for pivotally supporting the upper arm UA on the vehicle body frame are provided by an inward burring in the outer flange 3 at locations adjacent the base end of the upper arm UA.
As can be seen from FIG. 10, in the above-described upper arm UA made by pressing, the vertical dimension Ho of the outer flange 3 extending downwardly from an outer periphery of the body 1 and having the mounting holes 3.sub.1, 3.sub.1 is larger than the vertical dimension Hi of the inner flange 2 extending downwardly from an inner periphery of the body 1. As a result, as can be seen from FIG. 7, the centroid of a section of the upper arm UA is defined at a location closer to the outer flange than to the inner flange, and largely spaced apart at a central portion of the upper arm UA by a distance d.varies. from lines L, L (straight lines connecting the mounting hole 1.sub.1 and the mounting holes 3.sub.1, 3.sub.1) of application of a load transferred from the knuckle through the upper arm UA to the vehicle body frame.
Therefore, this known upper arm is disadvantageous in that an offset between the centroid and the load application lines L, L becomes large, resulting in a larger moment and hence, the outer flange 3 having the larger vertical dimension Ho and a high strength cannot effectively contribute to an increase in rigidity of the upper arm UA. If a sufficiently high rigidity of the upper arm UA is to be insured with the known upper arm, the plate thickness and corresponding weight of the body 1 must be increased.